Problem: Which of these numbers is prime? ${13,\ 26,\ 35,\ 49,\ 69}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 13 are 1 and 13. The factors of 26 are 1, 2, 13, and 26. The factors of 35 are 1, 5, 7, and 35. The factors of 49 are 1, 7, and 49. The factors of 69 are 1, 3, 23, and 69. Thus, 13 is a prime number.